1. Field of the Invention
The present invention relates to a heat-resistant coating composition and a heat-resistant coat making use of said composition, and more particularly to a heat-resistant coating composition and a coat consisting of said composition which has stable rust-proofness even after it has been exposed to any temperature within a temperature range of -50.degree. C.-650.degree. C. for a long period.
2. Description of the Prior Art
Recently, speed-up of the rotational speed of an engine and reduction of the weight and the size of an exhaust pipe in a motor-cycle, a motor tricycle, a motor car, etc. have advanced, and the surface temperature of an exhaust pipe has become high as compared to that in the prior art to such extent that it exceeds the limit temperature of 550.degree. C. of the heat-resistant rust-preventing paint used in the prior art and reaches 650.degree. C.
An exhaust pipe of a motor car engine is composed of an exhaust pipe section connected to an exhaust manifold or directly to cylinder heads and a muffler (silencer); the closer a portion of the exhaust pipe is located to the engine, the larger temperature change to which said portion is exposed, and at the rear end portion of the muffler which is remote from the engine, the temperature change is 100.degree. C. or lower. The condition of use of the exhaust pipe section close to the engine is especially severe, in a cold district it is used in the temperature range of, for instance, from an atmospheric temperature -30.degree. C. to about 650.degree. C. upon high speed rotation of the engine, and moreover, upon starting of the engine the temperature rises at a rate of about 600.degree. C. per minute and reaches the highest temperature after only about 70 seconds.
In addition, during running of a vehicle, pebbles, sand, mud, water, etc. splashed by wheels would collide with the surface of the exhaust pipe, hence an impact force or a thermal impact would act upon the surface, while during stoppage of the vehicle, the exhaust pipe is exposed to a corrosive environment caused by rain water, salty water or dew water produced at night, and so, corrosion of the exhaust pipe proceeds.
Heretofore, as a heat-resisting coating material for protecting an exhaust pipe, a silicone series heat-resisting paint has been used, and the composition of the paint consists principally of silicone resin or modified silicone resin, metallic zinc and inorganic pigment. A metal surface to which this composition is to be applied, is preliminarily roughened by subjecting it to a shot blast treatment, the above-mentioned composition is applied on the roughened surface to form a lower layer, then a composition consisting of silicone resin, metal oxides and inorganic pigment is superposed thereon, and thereby a heat-resisting coat is formed.
The principle of rust-prevention by means of this heat-resisting coat is such that in the low temperature range, invasion of water and salts which act as corrosive media to the surface of metal is prevented by the coat, and after heating, it is aimed to realize a sacrificial anodic effect of metallic zinc making use of a potential difference between the base metal and zinc. In the case of the heat-resisting coat in the prior art, there exists a temperature range where either of the above-mentioned rust-preventing effects appears poorly, and more particularly, a rust-prevention power after heated in the temperative range of 200.degree. C.-300.degree. C. is poor, and so, on the practically used exhaust pipe, often rust was produced at an early time.
As a result of seeking for the cause of the above-mentioned defects, the following has been clarified:
.circle.1 Even under the condition that the lower layer coating material has been heated up to 380.degree. C., metallic zinc particles contained therein are wrapped by carbide coating films of resin, hence assembled cracks (See FIG. 1) would appear while the rust-preventing effect relying upon the sacrificial anode does not appear, and so, a shielding effect for the corrosive media is lost.
.circle.2 The thickness of the coat is 60.mu. or less, and so, in the case where the height of the protrusions formed on the metal surface by the roughening treatment exceeds 40.mu., the thickness of the coating layer covering the protruded portions is remarkably reduced, hence the top portions of the protrusions are apt to be exposed, and thus rusting is promoted.